The effect of the gonadotropin, human chorionic gonadotropin (HGG) will be studied in the in vivo in situ vascularly perfused testis of the dog. The cytological appearance of the Sertoli cells, the sustentacular cells of the seminiferous epithelium, will be observed by light miroscopy and by electron microscopy of thin sections and freeze-fracture replicas. A correlation of cytological alterations of the Sertoli cells with their functional role in spermatogenesis will be based on the appearance of normal tissue as compared to HGG stimulated tissue. BIBLIOGRAPHIC REFERENCES: Connell, C.J. (1976). A scanning electron microscope study of the interstitial tissue of the canine testis. Submitted. Connell, C.J., and Connell, G.M. (1976). The interstitial tissue. In "The Testis" (A.D. Johnson and W.R.Gomes, eds.), Vol. IV. Submitted.